Molecular Motion Teacher Guide



Molecular Motion



Grade Level


Activity Name(s)

Active Transport

Diffusion and Osmosis

Being Prepared

Ideally, students would have one to one computer access where they are close to other students in order to discuss the activities. They may also work in groups of 2-3 if necessary.

Getting Started

Both of the activities in the unit are simulations so they only require a computer with internet access.

Suggested Timeline

This unit will take two days, or each activity needs about 35 minutes to complete so it is possible to do both in one block period but that could be a lot of rigor in one period as well as shortchange students a chance to reflect and debrief.

Thinking about the Discovery Questions

The molecular motion unit focuses on the random motion of molecules, how concentrations change based off this fundamental concept, as well as it's role in biological systems. This unit incorporates both physical and life science concepts.

The first activity's discovery questions, "What is the source of energy for actively transporting ions against their equilibrium concentrations?" allows students to investigate how ions (charged atoms) move against their concentration gradient.

The second activity's discovery question, "How do molecules and ions move into and out of cells?" will have students investigating how molecule concentration, water concentration, and molecule size affect diffusion and osmosis.


  • Atoms/molecules do not move, or if they do, they all move at the same speed.
  • Atoms/molecules are stationary until heated up.
  • Atoms/molecules will not naturally uniformly disperse in a system, that somehow they will not be found in certain parts (Example - atoms will be spread out in a system but not in the corners).
  • Atoms/molecules can somehow "see" where they're going and know where it will be less concentrated.
  • Diffusion is the process of atoms/molecules moving from a high concentration to a low concentration and osmosis is the diffusion of water.
  • They may think that energy is "stored" in bonds or released by breaking bonds, whereas energy is released when a weaker ATP bond is broken and the phosphate creates a stronger bond with another molecule, like glucose.

Learning Objectives

  • NGSS
    • Performance Expectations
      • HS-PS1-5. - Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
    • Disciplinary Core Ideas
      • HS-PS1: Matter and its Interactions
        • PS1.B: Chemical Reactions
          • Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy. (HSPS1-4),(HS-PS1-5)
    • Cross Cutting Concepts
      • Patterns
        • Students recognize that macroscopic patterns are related to the nature of microscopic and atomic-level structure. They identify patterns in rates of change and other numerical relationships that provide information about natural and human designed systems. They use patterns to identify cause and effect relationships, and use graphs and charts to identify patterns in data.
    • Practices
      • Developing and using models
        • Use and/or develop a model of simple systems with uncertain and less predictable factors.
        • Develop and/or use a model to predict and/or describe phenomena.
        • Develop a model to describe unobservable mechanisms.
        • Develop and/or use a model to generate data to test ideas about phenomena in natural or designed systems, including those representing inputs and outputs, and those at unobservable scales.
      • Planning and carrying out investigations
        • Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
        • Collect data about the performance of a proposed object, tool, process or system under a range of conditions.
      • Analyzing and interpreting data
        • Construct, analyze, and/or interpret graphical displays of data and/or large data sets to identify linear and nonlinear relationships.
        • Use graphical displays (e.g., maps, charts, graphs, and/or tables) of large data sets to identify temporal and spatial relationships.
        • Analyze and interpret data to provide evidence for phenomena.
      • Constructing explanations and designing solutions
        • Construct an explanation that includes qualitative or quantitative relationships between variables that predict(s) and/or describe(s) phenomena.
        • Construct an explanation using models or representations.
        • Construct a scientific explanation based on valid and reliable evidence obtained from sources (including the students’ own experiments) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
        • Apply scientific ideas, principles, and/or evidence to construct, revise and/or use an explanation for real- world phenomena, examples, or events.
      • Engaging in argument from evidence
        • Construct, use, and/or present an oral and written argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem.
        • Make an oral or written argument that supports or refutes the advertised performance of a device, process, or system based on empirical evidence concerning whether or not the technology meets relevant criteria and constraints.
      • Obtaining, evaluating, and communicating information
        • Communicate scientific and/or technical information (e.g. about a proposed object, tool, process, system) in writing and/or through oral presentations.
  • NSES
    • Life Science - The Cell
      • Cells have particular structures that underlie their functions. Every cell is surrounded by a membrane that separates it from the outside world. Inside the cell is a concentrated mixture of thousands of different molecules which form a variety of specialized structures that carry out such cell functions as energy production, transport of molecules, waste disposal, synthesis of new molecules, and the storage of genetic material.

Discussion: Setting the Stage

  • Do molecules move? If so, what makes them move?

    Yes, energy makes them move

  • What is concentration? Draw 2 boxes, one with a high concentration of atoms and one with a low concentration of atoms.

    The high concentration box should have many atoms and the low concentration box should have less atoms in it.

  • What is potential energy? Give 3 examples of potential energy.

    Potential energy is thought of as "stored" energy or the energy a system has the potential to "release". Examples - food or a neuron (chemical energy), a ball on a hill (gravitational), or a spring or rubber band (elastic).

Discussion: Formative Questions

Active Transport

  • As you "ride" your air molecule, what causes you to change direction?

    Collisions, either with a wall or another molecule.

  • Describe in order, what events have to happen for Calcium to be moved across the membrane.

    Calcium must first bind to the transport protein and then ATP must bind to the protein. Then this energy is used to pump the calcium ion across.

  • What causes the chemical energy (the left side yellow bar) to decrease? Where is the chemical energy stored?

    When an ATP is used and is converted to ADP + Pi. Chemical energy is stored in the ATP molecules.

  • What causes the voltage (electric potential - right side yellow bar) to increase?

    When Calcium ions are moved across the membrane. The larger the difference, the greater the potential (voltage).

  • Describe what must happen in the Electron Transport Chain in order for Hydrogen ions to move across the membrane.

    Multiple electron carriers must bind to the proteins in the membrane in order to "reduce" them which means 2 Hydrogen ions will move across for 2 electrons.

  • What do you notice about the oxygen molecules in the Electron Transport Chain?

    Oxygen moves across the membrane on it's own (diffuses) and it acts as the final electron receiver in the fourth protein (making water) which allows for another 2 hydrogen ions to pump across, further increasing the gradient.

Diffusion and Osmosis

  • What happens when there are two high concentrations of different molecules on opposite sides?

    Each molecule will balance out on each side; both molecules will reach equilibrium.

  • Why is there a lot of oxygen in the lungs as compared to far away from the lungs?

    Air enters our lungs directly through out mouth and trachea, which is where red blood cells pick it up.

  • How do red blood cells come in contact with oxygen in the lungs?

    There are blood vessels lining our lungs.

Discussion: Wrapping Up

Active Transport

  • Why is ATP necessary for active transport?

    Because energy is required to move molecules against their concentration gradient. (In the model it is a Ca pump)

Diffusion and Osmosis

  • What is the difference between diffusion and osmosis?

    Osmosis is the diffusion of water. Diffusion is the movement of atoms or molecules from a high concentration to a low concentration.

Additional Background

Active Transport

Voltage is the unit that describes electric potential. In this activity the potential increases as the concentration gradient becomes more extreme. This is because there is now a greater potential for ions to across the membrane towards a lower concentration. The more ions that are concentrated ("crowded") on one side, the more potential they have to move in a different direction due to their collisions. Most commonly discussed pumps are the ATP/Calcium pump and the Sodium/Potassium pump. There are 3 methods of active transport: unidirectional, symport, and antiport.

The Electron Transport Chain is generally studied during cellular respiration. The purpose of the chain is to create a large H+ ion difference between the inner matrix of the mitochondrion and the intermembrane space. This high concentration of H+ allows them to power the ATP-synthase pump by moving back across the membrane. Anything that prevents the series of reduction events in the chain will stop ATP synthesis, typically leading to death. For example cyanide is lethal because it interferes with the cytochrome oxidase by shutting it down.

Osmosis and Diffusion

Since water is so essential to biology it is imperative to understand osmosis. Osmosis allows all sea life to survive by controlling solute concentrations and preventing water loss by having water leave into the outside saltier water. (This is an example of hypertonic solutions)


Active Transport

  1. Describe how the chemical energy in ATP is converted into electric potential energy.

    Energy is released from ATP when the energy in the 3rd phosphate bond is transferred to another molecule. (Note: A common misconception is that energy is "stored" in bonds or released by breaking bonds. Energy is released when a weaker ATP bond is broken and the phosphate creates a stronger bond with another molecule, like glucose.)

  2. Does a high or low concentration of ATP cause high chemical energy?

    High, since chemical energy is a form of potential energy, more ATP means more molecules with stored energy.

  3. Describe in the Krebs Cycle the connection between the changes in hydrogen ion concentration and the changes in energy due to those concentration changes.

    Electrons are carried to membrane proteins. Electrons move across the membrane with hydrogen ions (reduction reaction) which creates a H+ gradient. The higher the gradient the greater the electric potential.

Diffusion and Osmosis

  1. Describe as many ways as possible how you know when a system has reached equilibrium.

    Net movement of atoms/molecules is equal in direction. On average, there is an equal numbers of atoms in each part of a system.

  2. Describe what will happen if a cell has an oxygen concentration that is higher outside of the cell than inside of the cell, as shown in the picture below.

    The oxygen will move into the cell because the oxygen will undergo more collisions when highly concentrated and will eventually move inside the cell.

Further Investigation

After these activities it would be a great opportunity to utilize student's conceptual knowledge by applying it to a wet-lab experience. Typical labs for osmosis is gummy bears in different solute concentrations, using potatoes in different concentrations of solute in water, or using dialysis tubing with starch and/or glucose. Diffusion can be viewed easily by dropping food color in a beaker of water and watching it spread out. All of these labs have great potential for inquiry as students can design investigations with different solute concentrations, temperature, mixing, etc.